1. Field of the Invention
The present invention is directed to a method for the removal of ATM cells from an ATM communications device in which a respective plurality of ATM cells are allocated to a common frame
2. Description of the Related Art
In a traditional packet communication system, a packet has a comparatively great and variable length. A system for the transmission of information in packets having fixed, predetermined lengths is referred to as an ATM (asynchronous transfer mode) system. Voice, video and data signals can be processed and transmitted in the same way with such a system. The individual packets are usually called cells. A cell header, whose information enables a switching or, respectively, allocation of the respective cell is respectively contained in the cells. A high-speed and broadband transmission with a transmission rate of more than 150 Mb/s is possible in ATM communication devices, particularly communication network devices.
In particular, the ATM cells have a length of 53 bytes for a broadband ISDN (Integrated Services Digital Network). The publication by M. DePrycker, “Asynchronous Transfer Mode”, 2nd Edition, London, Horwood, 1993, is referenced for further details about the structure of ATM cells.
One problem given ATM communication devices is the height of the transmission rate on a transmission link of the device when a back-up of ATM cells has formed at the transmission link. The problem is described in greater detail in the publication IEEE Journal on Selected Areas in Communications, Vol. 13, No. 4, May 1995, pages 633 through 641, “Dynamics of TCP Traffic over ATM Networks” by Allyn Romanow and Sally Floyd (referred to as IEEE 95 below). The article is concerned with ATM systems wherein a respective plurality of ATM cells are allocated to a common frame. When, for example, a cell of such a frame has been lost or damaged, it is undesirable that the further cells of the same frame are transmitted via a transmission link of an ATM device since the complete information of the frame would no longer arrive at the end of the transmission link. The ATM system would be unnecessarily burdened. Particularly given a back-up on the transmission link, it is therefore important to remove the further cells of the frame as, quickly and effectively as possible.
It is therefore proposed in the publication IEEE Network Mag., Vol. 7, No. 5, pages 26 through 34, September 1993, “Packet Reassembly during Cell Loss” by G. Armitage and K. Adams (referred to below as IEEE 93) to remove ATM cells of a specific frame at the respective arrival of an individual ATM cell at the end of a waiting list. In particular, such waiting lists serve for the administration of a sequence of ATM cells at the end and/or at the start of a transmission link. According to the method described in the IEEE 93 publication, which is referred to as partial packet discard (PPD below), the first and—when present—further cells of the frame that are already in the waiting list are not removed; rather, only all newly arriving cells of the frame are removed, with the exception of the last cell of the frame. PPD has the disadvantage that at least the first and the last cell continue to remain in the waiting list.
Waiting lists are usually organized according to the FIFO principle, in accordance wherewith the cell that arrived at the waiting list before another cell also in turn leaves it first. Under certain circumstances, however, the cells are divided into at least two priority classes, whereby cells of a higher priority are handled on a privileged basis.
The IEEE 95 publication discloses another method according to which all cells of a frame, from the first to the last cell are removed from the ATM communication device upon arrival at a waiting list. This method, called early packet discard (EPD below) has the advantage that no residual cells of a damaged frame or of a frame to be removed for other reasons remain, and, thus, the maximally possible space is available for other ATM cells. EPD, however, cannot be applied to frames whose first cell has already been added to the waiting list.